This invention relates to an exhaust gas particle sensor and in particular to a vehicle exhaust smoke sensor.
Increasingly legislation is being enacted to set limits on vehicle exhaust smoke, particularly from diesel engined vehicles.
It has been proposed in our co-pending patent application Ser. No. 9105731 that levels of smoke or other particles in vehicle exhausts could be sensed by measuring the opacity of the exhaust gases.
The opacity is measured by passing a light beam across a chamber through which the exhaust gases are flowing and measuring the reduction in the intensity of the light beam after its passage through the chamber.
For compression ignition engines, also known as diesel engines, the current UK exhaust smoke limit is defined as an absorption coefficient of 3.2 m.sup.-1.
A common type of opacity measurement system is where a light source and a main light sensor are placed on opposite sides of a sensing chamber. The intensity of a light beam from the light source incident on the light sensor is measured with only clean air in the sensing chamber, and then measured again with the exhaust gases in the sensing chamber. By comparing the two intensities the opacity of the exhaust gases can be calculated.
In such opacity measuring systems it is known to provide a monitoring light sensor which measures the intensity of the light beam generated by the light emitter before the light beam enters the sensing chamber. By comparing this measurement of intensity when air only is in the sensing chamber with a measurement of intensity of the light beam before the light beam enters the sensing chamber with exhaust gases in the sensing chamber, any changes in the intensity of the light beam generated by the light emitter can be compensated for. Generally where the light emitter is a light emitting diode and the sensors are photodiodes the monitoring light sensor is mounted adjacent the light emitter on a common header.
In theory such an arrangement should allow the opacity to be measured with great accuracy because any difference in the intensity of the light beam produced by the light emitter can be compensated for, leaving only the intensity differences produced by the opacity of the exhaust gases within the chamber. Furthermore, if the monitoring light sensor and the main light sensor are of the same type and processed by identical processing electronics it is possible to use the signals generated by the monitoring light sensor to compensate for changes in sensor efficiency or in the gain and efficiency of the processing circuits due to external parameters such as temperature. In practice however it has been found that where the light emitter and monitoring light sensor are placed on a common header and enclosed by a common enclosure light from the light emitting diode only reaches the photodiode after reflection from the enclosure and other parts of the optical system. As a result, otherwise trivially small relative movements of the parts of the optical system reflecting this light can cause sudden changes in the light intensity received by the photodiode. This is interpreted as a change in the light intensity produced by the light emitter and in attempting to compensate for this a change in opacity may be registered without any change in opacity having occurred.
This is a problem because it prevents an accurate measurement of opacity being produced.
Another problem encountered in optical opacity measurement systems is that external light sources impinging on the detectors can produce erroneous measurements of opacity because they interfere with measurement of light intensity. It is has been proposed to overcome this problem by employing a modulated light source and filtering the signals produced by the photodetectors to eliminate signals due to ambient light.
The problem with conventional filters is that their characteristics, including gain, are affected by the temperature coefficient of their components. Normal practice would be for the AC signal generated by the photodetector to be converted to DC and then sampled by an analogue to digital converter. This has the disadvantage that any residual noise in the signal will be added to the DC signal, thus producing an error.
This invention was made in an attempt to produce an exhaust gas particle sensor at least partially overcoming these problems.